We quantified hepatitis C virus (HCV) RNA at different times in plasma and peripheral blood mononuclear cells (PBMC) in 51 patients with chronic hepatitis C undergoing interferon-alpha2a (IFN-alpha2a) therapy. HCV RNA loads in plasma correlated with those in PBMC before and during the treatment (P<0.001). After treatment, a sustained response was observed in 19 patients (SR), a response followed by relapse in 9 (RR), and non response in 23 (NR). By univariate analysis PBMC HCV RNA load before treatment was lower in SR than in RR and NR (P = 0.003). In the 9 RR, HCV RNA disappeared in PBMC before or at the same time as in plasma and again became detectable in plasma and PBMC simultaneously or earlier in plasma. These results indicate that quantitation of HCV RNA in PBMC is not a useful parameter for the follow-up of treated patients.